1. Field of the Invention
The present invention relates to an organic electroluminescent device.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, many efforts and studies are being made to develop various types of flat panel displays, such as liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as a substitute for CRTs. Of these flat panel displays, organic electroluminescent displays (OELDs) are self-luminescent display devices. The OELD operate at low voltages and have a thin profile. Further, the OELD have fast response time, high brightness, and wide viewing angles.
The OELD is generally categorized into a passive matrix type OELD and an active matrix type OELD.
FIG. 1 is a schematic circuit diagram illustrating an active matrix type OELD according to the related art.
Referring to FIG. 1, the OELD includes a gate line GL and a data line DL crossing each other to define a pixel region P. A power line PL is formed in parallel with the data line DL.
In the pixel region P, a switching thin film transistor STr, a driving thin film transistor DTr, a storage capacitor StgC and an organic light emitting diode E are formed. An organic light emitting diode E includes a first electrode, a second electrode and an organic light emitting layer between the first and second electrodes. The first electrode is formed in each pixel region P and connected to the driving thin film transistor DTr. The second electrode is formed over all pixel regions and functions as a common electrode.
When an ON gate voltage is applied to the gate line GL, the switching thin film transistor STr is turned on, and a data voltage is applied to the data line DL. The data voltage passes through the switching thin film transistor STr and is applied to the gate electrode of the driving thin film transistor DTr. A current passing through the driving thin film transistor DTr is adjusted according to the data voltage applied to the driving thin film transistor DTr, and the current is applied to the organic light emitting diode E. The storage capacitor StgC stores the data voltage applied to the driving thin film transistor DTr while the switching thin film transistor STr is turned off.
FIG. 2 is a schematic cross-sectional view illustrating an OELD according to the related art.
Referring to FIG. 2, the OELD 1 includes first and second substrates 3 and 31 facing each other. The first and second substrates 3 and 31 are coupled to each other through a seal pattern 40 between peripheral portions of the first and second substrates 3 and 31.
On the first substrate 3, a driving thin film transistor DTr, and a first electrode 12 connected to the driving thin film transistor DTr are formed in each pixel region. An organic light emitting layer 14 is formed on the first electrode 12. The organic light emitting layer 14 includes red (R), green (G) and blue (B) organic light emitting patterns 14a, 14b and 14c in the respective pixel regions. A second electrode 16 is formed on the organic light emitting layer 14. The first and second electrodes 12 and 16 function to apply an electric field to the organic light emitting layer 14. The first and second electrodes 12 and 16 and the organic light emitting layer 14 form an organic light emitting diode in the pixel region.
The second substrate 31 functions as an encapsulation substrate and is spaced apart from the second electrode 16.
As described above, the organic light emitting layers emitting red, green and blue lights are formed in the respective pixel regions. The organic light emitting layers are different in lifetime according to their material properties. Particularly, the blue organic light emitting layer has the shortest lifetime. Accordingly, when the lifetime of the blue organic light emitting layer ends, the lifetime of the OELD also ends. In other words, the lifetime of the OELD depends on the lifetime of the blue organic light emitting material.
Further, the organic light emitting layer emits light having a specific wavelength according to its material property, and intensity and color of the emitted light are determined by passing through several layers. This is a mechanism for light emission from the light emitting diode. Since several mediums forming the several layers on the light path have respective predetermined refractive indexes, light reflectance and transmittance are determined. By using this, light chromaticity and intensity may be optimized. However, the related art OELD does not have the optimized configuration, light efficiency is relatively reduced.